The operation of coating outer surfaces of workpieces which are readily accessible by means of a plasma gun assembly known in the art usually can be performed without any problems. However, if inner surfaces of cavities have to be coated by using a plasma gun assembly known in the art, e.g. the inner surfaces of bores, channels, tubes and the like, various problems and difficulties arise.
One of the main problems in coating inner surfaces of cavities is the length of the bore or channel to be coated. As the connector portion of a known plasma gun assembly usually is much bigger then the plasma gun shaft member and the plasma gun head member mounted at the end of the shaft member, it is not possible to introduce the entire plasma gun assembly into the bore ore channel to be coated on its interior surface. In order to provide for a plasma gun assembly which is small and easily maintainable, suitable for short bores and channels, and for a plasma gun assembly usable for bores and channels of greater length, the design of the plasma gun assembly must be correspondingly adapted, at least as far as the portions thereof are concerned which are introduced into the interior of the bore or channel to be coated.
The outer diameter of a plasma gun assembly, particularly the diameter of its shaft member and its head member located at the end of the shaft member, determines the minimal size of the bore or channel whose interior surface has to be coated. In other words, the smaller the plasma gun head member and the plasma gun shaft member are, the smaller can be the diameter of the bore ore channel to be coated.
In order to provide for a homogeneous coating, particularly of angled and tortuous portions thereof, the plasma torch created by the plasma gun head member preferably should escape from it radially with respect to longitudinal axis of the plasma gun assembly.
A further problem is the heating-up of the parts and portions of the plasma gun assembly which are in the interior of the bore ore channel to be coated during the coating operation. It is well known in the art that temperatures in the region of 10'000.degree. C. can occur during a coating operation by means of a plasma gun assembly. This problem is even much more serious if the coating operation is performed under conditions in which the ambient pressure is less than the atmosphere pressure, particularly under vacuum or near vacuum conditions, since in this case a blowing-in of air or carbon dioxide is not possible to cool the hot parts of the plasma gun assembly as is possible if the coating operation takes place under atmospheric conditions. In order to avoid a damage of the parts and portions of the plasma gun assembly under atmospheric condition and particularly also under near-vacuum conditions, an efficient cooling of the plasma gun shaft member and the plasma gun head member must be provided.
In coating of narrow tubes and similar workpieces, a further problem to be considered is the electrical insulation of the plasma gun head member. Particularly in the case where a transferred arc is used, the shortest path thereof often being not identical with the course of the desired path between the cathode and the surface to be coated, for instance the inner wall of a tube, great care must be taken that the plasma gun head member is provided with a good insulation all over its circumference. In plasma gun assemblies known in the art, there is a danger that an undesired transformation of the plasma torch to the workpiece can take place if the electrical insulation of the plasma gun head member is damaged or impaired by the precipitation of dust, particularly if the plasma gun assembly is operated under vacuum conditions. Thus, the plasma gun assembly and particularly the plasma gun head member should be designed in such a way that the electrical insulation of the head member prevents an undesired transferring of the plasma torch to the workpiece surface to be coated.